AI email editing now supports section-level edits

Instead of regenerating an entire email template when you want to make a change, you can now click any section in the preview to select it and prompt the AI to edit only that part. The selected section stays highlighted while you work, and the chat clearly shows which section is active. You can exit the selection anytime with an X to return to full-template editing. This approach makes AI edits more predictable and reduces the risk of undoing work you're happy with.